M. Ussow’s Zooloyico-Embryological Investigations. 103 
larged cells of the granulosa and the ovicell, which has been 
pushed by the folds to the superior pole of the originally round 
Graatian follicle, are abundantly provided with nourishment. 
At this time—that is to say, in the period of the “ foldings” 
Kélliker)—the cells of the epithelial membrane begin to secrete 
the fluid, fatty, transparent nutritive vitellus. Consequently 
the fold-formation of the granulosa only serves for the tempo- 
rary enlargement of the imner surface of the Graafian follicle, 
which secretes the nutritive vitellus. In this state each 
Graafian follicle may be regarded as an independent gland. 
Of the vitelline membrane (chorion*) there is at this time not 
the slightest trace; so that the description of the nature of the 
so-called “ folding-process”’ as given by other observers T 
proves to be very superficial and erroneous (of which I have 
tully convinced myself). The chorion is formed subsequently, 
after the nutritive vitellus is completely secreted and the 
ovum has attained the limit of its perfect development. The 
chorion, which is at first fluid and viscous, is, indeed, nothing 
but a secreted product of the granulosa of the Graafian follicle, 
which may be proved by the fact, among others, that at the 
commencement of its formation, especially at the superior 
somewhat acuminated pole, its composition of several thin 
superimposed layers may be distinctly observed. At the same 
time there is formed at the above-mentioned thickened part of 
the chorion, in a manner which, I must confess, is still 
obscure to me}, its tubular micropyle, more or less widened 
and funnel-shaped in its upper part. This I have found in all 
the above-mentioned species and groups of the Cephalopoda. 
At a very early stage of development the Graafian fol- 
licle gradually changes its spherical form, and acquires the 
shape of an egg pointed at the upper free pole. The enclosed 
ovum follows in its form that of the Graafian follicle. The 
primitive ovicell with the nucleus (=germinal vesicle) moves, 
as already stated, to the upper pole of the Graatian follicle, 
which is now pretty acute (Loligo, Sepiola, Argonauta), and 
the granulosa of which has scarcely any folds and appears 
quite smooth at this part. Here, therefore, is the finely gra- 
nular protoplasm of the primitive ovicell ; and by this means 
* Kolliker (/. c. p. 15) and other observers (Klassen und Ordn. Bd. 11. 
p- 1405) quite erroneously take the external pluristratified capsule of the 
Cephalopod ovum for the chorion, and the true chorion (formed within the 
Graafin follicle and always furnished with a micropyle) for the vitelline 
membrane. 
+ Kolliker, . c. pp. 2-15; Brandt, Mediz. Zool. Bd. 11. p. 3800, Taf. xxxii. 
fig. 27; Owen, Memoir on the Pearly Nautilus, p. 42. 
{ Where the micropyle is situated there are no folds (“ free space,” 
Kolliker), and the membrana granulosa there forms a thin layer. 
